US 7029375 B2
The retaining ring has a plurality of slurry channels wherein each alternate channel is recessed away from the inner circumference of the pad contacting surface forming a recess which extends upward from the bottom surface sufficient to prevent contact of the retaining ring with the polishing pad. Each recess curves towards the inner circumference of the retaining ring in a manner to form a rounded tab, tangent to the inner circumference of the retaining ring, and meeting the inner circumference at the exit end of an adjacent non-recessed slurry channel. The total effective contact length of the ring with the wafer edge is about one-tenth of the wafer perimeter. This is sufficient to properly contain the wafer during polishing and provides a large area of undistorted polishing pad at the wafer edge. By adjusting the operating pressure of the polishing head, it is possible to obtain polishing rates at the wafer edge which are larger or smaller than the overall wafer polishing rate.
1. A retaining ring for a rotatable chemical mechanical polishing head comprising:
(a) an inner peripheral surface;
(b) an outer peripheral surface;
(c) a lower surface adapted to contact and depress an upper surface of a polishing pad during chemical mechanical polishing of the lower surface of a substrate contained within said inner peripheral surface during chemical mechanical polishing; and
(d) said lower surface having a even numbered plurality of evenly spaced slurry channels, originating at said outer peripheral surface and angled toward said inner peripheral surface, each of said plurality of evenly spaced slurry channels being radially angled in the direction of rotation of said polishing head, and wherein said plurality of evenly spaced straight slurry channels are further arranged in an alternating sequence of long and short channels, proceeding around the perimeter of said retainer ring, wherein each one of said long channels extends from said inner peripheral surface to said outer peripheral surface and each one of said short channels extends from said outer peripheral surface to a corresponding cavity of a plurality of cavities formed in the lower surface, each one of said plurality of cavities extending from said inner peripheral surface towards said outer peripheral surface for a radial depth and a height above said lower surface sufficient, to prevent depression of a subjacent polishing pad under the edge of a retained wafer adjacent to said cavity, each of said plurality of cavities being further bounded laterally by non-recessed regions which form segmented tabs through which adjacent long channels pass, there being as many segmented tabs as there are long channels, each of said segmented tabs having a rounded edge, tangent to the inner circumference of said retaining ring, meeting said inner circumference at an end of and symmetrical with a non-recessed long channel and whereby each said segmented tab has an effective contact width at said inner peripheral surface against a wafer enclosed therein.
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(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to the chemical-mechanical-planarization of semiconductor wafers.
(2) Background of the Invention and Description of Previous Art
The fabrication of integrated circuits not only involves the forming of semiconductor devices within the surface of a semiconductor wafer but also the creation of a complex network of wiring interconnections which comprise the electrical circuitry of the completed chip. These interconnections are accomplished by the alternate deposition of thin layers of conductive and insulating materials over the semiconductor devices. Each conductive layer is patterned by photolithographic techniques to form the wiring design for that level. This patterning process produces a surface with topological features, which, if no steps were taken, would replicate itself in each succeeding layer.
Conductive layers, usually metals such as aluminum or copper, are deposited by physical-vapor-deposition (PVD) techniques such as vacuum evaporation or sputtering. These methods do not provide conformal coverage and the presence of topological features on the surface onto which they are deposited result in non-uniformities in thickness and other problems related to the shadowing effects of non-planar surfaces.
Frequently two to four levels of interconnection metallurgy are required to form the required circuits. In order to provide a planar surface for each level of metal, various methods have been used to planarize the insulator surface. Glasses which flow when heated are commonly used to accommodate the first layer of metallization. Subsequent levels where elevated temperatures are no longer permissible, employ layers of materials which are deposited as liquids and then cured to form solid layers. Such layers of spin-on-glasses and organic polymers provide an improved local surface planarity. Subsequent reactive-ion-etching(RIE) removes the polymers and translates the new surface to the insulating layer.
Although these methods can provide to local planarization, they are impractical for the large wafers used present day technology because they cannot provide planarization over a large area. This is because the integrated circuit chips themselves contain discrete regions of different topological complexity such as memory arrays located within regions of logic circuits.
Recent years have seen an increased interest in the old technique which is used to provide the wafer with a planar surface in the first place—chemical-mechanical-polishing(CMP). At first thought, CMP is a seemingly crude method for dealing with dimensions of the order of hundredths of a micron. However, because of it's uniquely global planarizational ability, the CMP technique has been diligently and painstakingly refined and perfected for in-process planarization over the past decade. The main thrust of this effort has been dedicated to the development of highly sophisticated polishing machines. As a result, the CMP technique is now widely used for global planarization of in-process semiconductor wafers in the fabrication of integrated circuits.
Modern, high precision CMP machines are described, for example, by Hempel, U.S. Pat. No. 5,597,346, Kim, U.S. Pat. No. 5,695,392, Zuniga, et. al. U.S. Pat. Nos. 6,132,298 and 6,361,420 B1, Yi, U.S. Pat. No. 6,146,260, and Lin, et. al., U.S. Pat. No. 6,183,350 B1.
An example of a simple CMP polishing machine as described by Lai, et. al., U.S. Pat. No. 6,224,472 B1 is shown in
The polishing rate at wafer edge is usually different from that on the rest of the wafer. In addition, the polishing rate profile at the wafer edge does not vary much with different process parameters or materials being polished. Typically, the polishing rate at less than 5 mm. from the wafer edge is always lower than that at the wafer center. This puts a limitation on the thickness uniformity especially at the wafer edge that can be achieved by CMP and hence makes the wafer edge region unsuitable for use in integrated circuits, and decreases the yield.
It would be desirable to be able, by polishing parameter adjustments, to regulate the polishing rate in the edge region of the wafer so as to achieve a uniform polish rate over the entire wafer and thereby extend the overall usable area of the wafer. It would be further desirable to be able, by polishing parameter adjustments, to adjust the polishing rate in the edge region of the wafer so as to polish the wafer edge region at a significantly higher or a significantly lower rate than that over the main portion of the wafer. With this capability, either surface planarity and/or surface layer thickness uniformity can be extended further out into the edge region.
Zuniga, et. al., '298 and '420, describe a polishing head structure which provides adjustable pressure during polishing to the wafer in order to cause even polishing. A first pressure is applied to a center portion of the wafer through a membrane and a second pressure is applied to a perimeter portion of the wafer through an edge load ring which is more rigid than the membrane. In Zuniga, et. al., U.S. Pat. No. 6,436,228 B1, there are shown a wafer substrate retaining ring which provides a plurality of discrete points of contact between the ring and the wafer. However, these various designs do not address the delivery of slurry under the wafer.
Notable progress has been made in retaining ring design, primarily with regards to the uniform and controllable flow of slurry past the retaining ring to the underside of the wafer. Chiu, et. al., U.S. Pat. No. 5,944,593 shows a retaining ring which features a plurality of straight channels for the delivery of a slurry and an added circular groove which provides an internal interconnection of all the straight channels. This design provides a buffering effect to the delivery of slurry. Lin, et. al., U.S. Pat. No. 6,062,963 cites the use of slurry delivery channels which are tapered to even out the slurry flow at the edge of the wafer. Glashauser, U.S. Pat. No. 6,419,567 B1 also provides an improved slurry delivery system through the retaining ring with a central circular channel in the base of the retaining ring which is fed with slurry through two inlets and which distributes slurry to the wafer through a plurality of inner channels. Pham, et. al., U.S. Pat. No. 6,447,380 B1 like Glashauser, also addresses modifications of the retaining ring to improve slurry delivery to the wafer.
While most of the retaining ring design effort has directed at slurry delivery, little has been given towards edge control. The Mirra-Mesa™ polisher manufactured by Applied Materials Corporation of Palo Alto, Calif. permits the application of a first pressure to the backside of the wafer during polishing through a flexible membrane (designated as membrane pressure or MP). A second pressure, designated as retaining ring pressure (RRP), is applied to a retaining ring which surrounds and confines the wafer. A third pressure, referred to as inner tube pressure (ITP), permits the application of an additional controllable downward pressure to a circular region near the edge of the wafer by means of an inflatable tube, between the membrane and the wafer. With this capability, the present inventors have found that, using the conventional retaining ring shown if
In response, the present inventors have developed a new retaining ring design which permits edge polishing rates which can be made significantly greater or significantly less than the mean overall wafer polishing rate depending on the choice of ITP and RRP. None of the prior art provides the capability of such wafer edge polishing rate adjustment by a simple in-situ pressure change. In view of the above the improved retaining ring structure cited by the present invention helps to minimize deformation of the polishing pad and thereby is capable of providing good thickness uniformity and/or surface planarity over a greater portion of the wafer than was previously possible.
It is an object of this invention to provide a design of a CMP wafer retaining ring which provides a uniform slurry delivery to a wafer contained therein.
It is another object of this invention to provide a design of a CMP wafer retaining ring which provides secure wafer containment with minimal wafer edge contact.
It is yet another object of this invention to describe a design of a CMP wafer retaining ring which provides minimal polishing pad distortion at or near the edge of a wafer contained therein when downward pressure is applied to said retaining ring.
It is yet another object of this invention to describe a design of a CMP wafer retaining ring which provides the capability of instantaneously manipulating the polishing rate of the edge region of a wafer by adjusting both the retaining ring pressure and the wafer pressure against the polishing pad to make the edge polishing rate either significantly greater than or significantly less than the main wafer polishing rate.
These objects are accomplished by a retaining ring design having a plurality of straight slurry delivery groves, angled in the direction of rotation of said ring wherein each alternate channel is recessed away from the inner circumference of the bottom, pad contacting, surface, of said retaining ring by a recess which extends upward from the bottom surface only sufficiently to prevent contact of the retaining ring with the polishing pad in the area of the recess. Each recess curves outwardly towards the inner circumference of the retaining ring in a manner to form a symmetrical segmented tab with a rounded edge, tangent to the inner circumference of the retaining ring, and meeting the inner circumference at the exit end of an adjacent non-recessed slurry channel.
Each segmented tab contacts the enclosed wafer over a distance, which includes the width of the slurry channel exit, which is about 11.5 percent of the contact distance of a comparable un-recessed retaining ring.
It is another object of this invention to provide a method for improving the thickness uniformity and/or surface planarity of a surface layer on a wafer, wherein the surface layer initially exhibits radially symmetrical non-uniform thickness or planarity between its edge region and its interior region.
This object is accomplished by first depositing or otherwise forming a desired surface layer to a thickness which is greater than or equal to the desired final thickness in both edge and interior regions. The wafer is then mounted on the head of a CMP apparatus which is fitted with a wafer retaining ring having a recessed face as cited supra and will be described in greater detail in the first embodiment of this invention. The CMP apparatus must also has the capability of selectively and independently applying downward pressure (ITP) onto back of a wafer edge, (MP) onto the wafer bulk, and (RRP) onto the retaining ring. Prior to said mounting, the CMP apparatus must first have been calibrated to determine the polishing rates of a corresponding surface layer on a calibration wafer as a function of the ITP and RRP. It will be found that under one set of parameters the edge polish rate will be significantly greater than that over the interior region of the wafer. For another set of parameters the edge polish rate will be significantly less than that over the interior region of the wafer.
The surface layer of the wafer is then polished, using a set of parameters which will polish the thicker region at a faster rate than the thinner region. A point will then be reached when the edge region and the interior region of the surface layer are essentially of the same thickness or the upper surface of the layer is essentially planar. If upper layer overall thickness uniformity is sought, the upper surface is not necessarily planar. However, if the wafer surface was planar overall before deposition of the surface layer, both layer thickness and surface planarity will be realized. Finally, if necessary, the polish parameters are the switched to a set which gives the same polish rate for the edge region and the interior region. The surface layer is then uniformly polished down to the final desired thickness.
In a first embodiment of this invention a design of a retaining ring for a CMP head is described. The embodiment will be geared towards polishing an oxide layer on a 200 mm. diameter standard silicon wafer. The head of the CMP machine will therefore be discussed in terms of this size wafer. However, the embodiment is not limited to a 200 mm. wafer but may be applied to any wafer size.
Experimental data will be presented in which the performance of the novel retaining ring is compared to the performance of the prior art retaining ring design shown in
In order to overcome the distortion of the polishing pad by the retaining ring at the edge of the wafer, and thereby improve polishing rate control in the edge region, the retaining ring design was modified to recess the area of contact of the retaining ring away from the wafer edge. Referring now to
The most desirable configuration of the recessed edge retaining ring of the present invention is shown in isometric view in
Experiments were also performed on segmented contacts which were not rounded but had a square off outer face straddling the slurry channel. However, the squared corners were found to chip away during polishing and therefore this option was not pursued.
The improved retaining ring 122 illustrated in
Experiments were performed to evaluate the improved retaining ring design. For these experiments a standard retaining ring of the type shown in
With this arrangement the polishing pad is free of distortion by the retaining ring, as illustrated in
The wafers used in the polishing experiments were conventional 200 mm. diameter silicon wafers, each with a 10 micron thick blanket layer of silicon oxide deposited on their surface by chemical vapor deposition (CVD). The thickness of the oxide layer on each wafer was measured at many locations on the wafer before and after polishing using the Nano9000 thickness measurement capability of the CMP polisher. The tool is capable of measuring thicknesses out to about 99.5 mm. from the wafer center which is essentially the very edge of the wafer. The polishing rate was then plotted for the many measurement locations as a function of distance from the center of the wafer. The polishing slurry and pad were the ILD1300 slurry and the OXP4100 polisher pad respectively.
Similar trends of polish rate profiles were observed for standard retaining ring and improved retaining ring when the RRP is fixed at 4.5 psi. These results are presented for the standard retaining ring in
Compared to the standard retaining ring design (
For standard Retaining ring, under RRP=4.5 psi and ITP=5.5 psi, the edge polish rate seems to be higher than the average polish rate according to
The differences in wafer edge polish rate profile between standard Retaining ring and improved Retaining ring suggests that the presence of empty space between the contact segments changes the interaction between wafer and pad under different ITP and RRP.
In the standard retaining ring arrangement, there is pad deformation along the edges of the retaining ring due to the RRP. In addition, the pad deformation is not constant along the radial direction towards wafer center. Such pad deformation causes a small separation between wafer surface and pad near the Retaining ring edge as shown in
The improved Retaining ring structure is able to provide a complete range of polish rate profile for wafer edge. More specifically, it allows wafer edge polish rate to be adjusted either higher, equal, or lower than the polish rate at the wafer center region. On the other hand, the standard retaining ring always gives low wafer polish rate at wafer edge. The total length of contact segments and the empty space between them on the improved retaining ring plays an important role in allowing different degrees of contact between the polishing pad and the wafer surface around the edges which, in turn, leads to an improved wafer edge polish capability. The improved retaining ring structure is generally applicable in other polisher heads that uses a similar retaining ring to keep the wafers in position during polishing.
While the trend in polishing rate for the standard retaining ring is always downward towards the very edge. This is generally not found for the improved retaining ring at ITP=5.5 psi and RRP=4.5 psi. However the data in
The improved retaining ring allows a flat thickness profile to be achieved regardless of the incoming thickness profile (flat, edge thick, or edge thin). In addition, since the polish rate profiles can be varied easily through the retaining ring pressure and the inner tube pressure, polishing conditions can be easily adjusted during polishing to achieve the desired end profiles. The improved retaining ring provides a simpler and cheaper option for better wafer edge polishing rate as well as an increase is useable wafer area.
In a second embodiment of this invention, a method for using the improved retaining ring, described herein, is presented. Referring to
Wafer 40 is next mounted on the head of a CMP polishing machine such as the Mirra-Mesa™ polisher fitted with a Titan-1 polisher head and an improved recessed face retaining ring described by the first embodiment of this invention. The polisher also must have the capability of selectively and independently applying downward pressure (ITP) onto back of a wafer edge, (MP) onto the wafer bulk, and (RRP) onto the improved retaining ring. Using a ILD1300 slurry and the OXP4100 polisher pad, a platen speed of about 110 rpm, a head speed of about 104 rpm., a membrane pressure of about 3 psi., a slurry flow of about 100 ml./min., a ITP of about 4.5 psi. and a RRP of about 4.5 psi., the oxide layer 42 is polished until the surface becomes essentially planar as shown in
Referring back to
The second embodiment describes a method for planarizing and polishing a layer to a desired thickness wherein the initial surface had an edge region which is thinner than the interior region. It should be understood that, for the reverse case, wherein the edge region is thicker than the interior region, the first set of parameters to use to achieve a planar surface would be of lower ITP such as reflected in
While the embodiments of this invention are described for a 200 mm. diameter wafer, it should be understood that they can be applied to other wafer sizes as well.
While a silicon oxide layer was used to illustrate the second embodiment of this invention, the principles and techniques of the embodiment could be applied to other material layers as well.
While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.